Electron stream amplifier tube



4 Sheets-Sheet l A. V. HAEFF ELECTRON STREAM AMPLIFIER TUBE April 3, 195e Filed April l2, 1952 April 3, 1956 A. v. HAE-FF 2,740,917

ELECTRON STREAM AMPLIFIER TUBE Filed April l2, 1952 4 Sheets-Sheet 2 i i u i T l I 1 n g. i i TI l l i 1 'l 1 11 "Q :Isl l *i wm l l 1 1i w Mq .t i E I 1l i I af a I nvr/Emme.I E ,44m/MW K #4f/:5 BY l ,drm/Mfr.

April 3, 1956 A. v. HAEFF 2,740,917

ELECTRON STREAM AMPLIFIER TUBE Filed April l2, 1952 4 Sheets-Sheet 3 Q 'Q N t jira-A4.

IN VEN TOR.

April 3, 1956 A. v. HAEFF 2,740,917

ELECTRON STREAM AMPLIFIER TUBE Filed April l2, 1952 4 Sheets-Sheet 4 ,lfm/Min United States PatentO' t, ELECTRON STREAM AMPLIFIER TUBE. l AndrewV.` Haelf, @Pacific Palisade,s,. Calif., assignor, by

mesne assignments, to Hughes Aircraft Company, a corporation of Delaware l y p l j Application April 12, 1952V,`Serial No. 282,000 12 Claims. (Cl. S15-3.6)

`This `invention' relates to electron stream amplifiers, and more'particularly to the electronstream amplifiers which are capable of amplifying microwave energy by; meansof the interaction of the electrons of-` arnodulate'dV electron -stream with electromagneticlields `produced byA currents induced in a medium contiguous tothe Vst-ream by the modulateclelectronV stream in a medium contiguou'stothestr'eam.VV V

In a simple form, the device, in addition to the usualy electron gun and collector electrodes, 'contains three *sections. The'first section is a relatively short input struc-i turewhose function is to transform signal energy--into modulations' of the electron stream. p j

'Tlief'se'cond section is anon-propagating structure" includiiigvw'alls .having a Surface that vpresents reither ai resistivelcapacitive', resistive, resistive-inductive or indue-1.1 tive impedance'to the stream. Non-propagating is to beVV construed to mean that'the structure will very `highly ati tenuate electromagnetic waves at the signal vfrequency in* the absence 'of an electron stream. In its simplest formi'1 this structurecomprises along piece of glass tubinghav-` ing a' resistive coatingonits inner surface( VvThis struc ture, with the resistiv'e coating on its inner surface, and other"versions" 'of this "structure,'"which` will be described? more fully later in this specification, all will beI generallyA referred to as an impedance member in this specifica` tion. It is, at this time, premature to define fully the meaning of this term, butitwillbe Ydefined `more fully later in this specification, where its nsig1iific`an`ce will`become more apparent in` the light of the function"` and results produced by this member. It is to be noted, howevergthat the impedance member in its simplestV form does not present a purely` 'resistive impedance to the elecf'* tron stream but"rather a resistive-capacitive'impedance due to" inherentV spatial 'capacitance' along "its surfabel,` This capacitance is parasitic in nature since it acts las "a low impedance path for the currents induced inthe walls of the member. `To counteract this effect, some of the versions of the impedance member have an artificially inf trduced inductance to neutralize the spatial capacitance. When this is the case, then the impedance presented to` the electron stream by the walls of the member may become, for all practical purposes, resistive, resistive inductive, or inductive. VThe inductance presented tothe electron stream by the walls of the member may be increased in several ways, for example, by usinga ferritic material or by scattering highly conductive dipoles throughout a dielectric material. As will be pointed out later, in order to obtain maximum gain, the inductance should cancel the effect of the spatial capacitance at the center operating frequency; in this case, the resistance offered by the'resistive coating should be as high as'pos'- sible consistent with maintaining the surface contiguous to the electron stream at a'uniform quiescent potential. The original modulations of the stream are amplified'- through the process of interaction between the modulated electron stream and the electric fields produced by the 2,740,917 Patented Apr. 3, 1956 ice currents induced in the walls of the impedance member bythe modulations of the electron stream.

The third section of the device is a simple output structure where the amplified signal energy in the stream is `converted into a useful output signal.

In the electron stream type tube disclosed in this specification, a microwave input signal is made to modulate an electron stream which produces a series of charge condensatic'nsV within the electron stream constituting a space charge wave propagated by the electron stream and corresponding to the input signal. The electron stream propagatingthe space charge Wave is directed along con-y tiguous -to the surface of the impedance member. Charge'fcondensations propagated by the electron stream produce corresponding electric field variations in Athe stream'- and" in 'the contiguous walls of the impedance member- These electric fields induce currents in the por-` propagates along the stream. In the present invention,

the` pavs'siu'g ofthe electron stream inA the proximity of the impedance member is actually an approximation of passing the feledtrons through a medium `havingisuitable` dielectric? properties similar to the dielectric properties of i the contiguous surface of the impedance member.

Amplification of the space charge wave, in passing in the proximity of the surface of the wall of the impedance member, is accomplished by a partial transfer of the average kinetic energy of the stream electrons to the space charge vWave there are no critical requirements imposed on thevelocity of the electron stream bythe characteristics of the impedance member. After the stream elec trons have progressed past the member; the microwave energy is removed from the electron stream by means of an output circuit coupled to the electron stream.

Some advantages of the new device can be brought out by comparing the characteristics of the more convenfv tional' amplifiers, such as traveling wave tubes, tov the characteristics of the new device. In conventional traveling waveV tubes, amplification of microwave signals'is obtainediby applying the signal to a wave-guiding struc-'f ture, .capableof Vpropagating a slow traveling electro-A magnetic wave. An electron stream is projected contiguous to' this slow-wave structure at a velocity slightly greater than* thephase velocity of the waves on the struci ture. The electron streamy interactsv with the traveling wavein such a'manner that energy is transferred from the electron stream to the traveling wave, thereby increasing q the amplitude of the wave as it travels along the structure,' resulting' in aconsiderably greater signal at the output as ,p

In order to obtain high compared to the input signal. energy gain," the .waveguiding' structure of a traveling wave tube has to'be Vmade with Vconsiderable precision Y since the phase velocity vof'the electromagnetic Waves.- should'remain substantially constant along the lengthof 7 thefstructure 'and infiXed relationship to the velocity of electrons projected contiguous tothe structure.` At oper-4 ating voltages convenient to use, which are normally of the'order of SOO-3000 volts, it is somewhat diicult to achieve the necessary degree of uniformity of wave-A guiding structures designed to operate in the kilomega-` cyclelrange. This is-so because of very smalltoleran'ces required inthe fabrication of the slow-wave structure. Con ,ferable uniformity in electron -velocity is also re'- quiredV in ordertoachieve high gain in a traveling wave tube. This is again diiiicult to achieve due to the presence of transverse electric fields in electron-accelerating structures and due to space charge forces produced within the stream itself.

The present invention obvates the necessity of fabricating long and very precise Waveguiding structures, since the amplifying section may be a relatively simple member consisting, for example, of a piece of straight glass tubing having a resistive coating on the surface contiguous to the stream. This simple structure makes it possible to build electron stream type amplifying tubes to operate at significantly higher frequencies than is now possible with conventional traveling wave tubes. In addition, the amplification mechanism of the present invention does not require a critical adjustment of electron velocity in order to achieve high amplification. This makes it possible to use a high density electron stream, even though there may be a considerable variation in the velocity of the electrons across the stream cross-section. Another advantage of the present invention is that, in the amplifying section of the tube, signal energy is carried almost entirely by the electron stream itself and hence can propagate only in the direction of electron travel. This unidirectional energy propagation makes it possible to obtain very high gain in a single tube without danger of oscillation because of the absence of feedback through the amplifying region of the tube. Such energy feedback effects are present in conventional traveling wave tubes and are usually avoided only by artilicially loading the waveguiding structure resulting in a considerable sacrifice in gain. The freedom from uncontrolled oscillation in the present device also removes the stringent requirement on the impedance matching of input and output circuits over the wide range of frequencies which is usually required in conventional traveling wave tubes and which is particularly difficult to achieve in traveling wave tubes designed for operation at very high frequencies.

In addition to the foregoing advantages, the disclosed impedance wall tubes also provide gain and have a frequency response comparable to that of prior art ultra high frequency amplifiers without the accompanying structural complications.

It is, therefore, an object of this invention to provide a method and apparatus for amplifying a space charge wave by passing a modulated electron stream contiguous to a surface of an impedance member.

Another object of this invention is to provide an electron stream amplifier tube capable of amplifying microwave signal energy of broad bandwidth, the amplification being accomplished by passing a signal-modulated electron stream contiguous to the surface of a member that presents an impedance to the electron stream whereby the relationship between the velocity of the electron stream and the characteristics of the member are such as to permit the operation of the tube over a wide range of electron stream velocities.

A further object of this invention is to provide a stream type amplifier tube wherein the walls of an impedance member present an inductive admittance to the electron stream by means of .imbedded inductive elements within the wall structure thereby canceling the inherent spatial capacitance of the surface of the walls.

A still further object of this invention is to provide a stream type amplifier tube wherein the admittance presented to the electron stream by an impedance member including a resistive surface is made resistive-inductive in nature by incorporating material therein that exhibits a higher permeability at the operating frequency than that of free space, thereby to present a higher irnpedance to the electron stream than is possible with a resistive surface alone, due to its inherent capacitance.

Still another object of this invention is to provide a stream type amplifier tube having an impedance member which produces a signal-amplifying effect on a modulated electron stream, the impedance member having a resistive-inductive surface positioned contiguous to the electron stream between the input and output circuits of the tube.

An additional object of this invention is to provide an electron tube including an impedance member having an inductive reactance wall capable of amplifying microwave signals propagated by an electron stream, the tube being readily adaptable to a variety of input and output circuits for coupling to and from the electron stream of the tube, these circuits taking such forms as a beammodulating grid, resonant cavity, a short helical waveguide section, or a short helical coil.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings, in which several embodiments of the invention are illustrated, by Way of examples. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Figs. 1, 2, 4, 7 and 8 are diagrammatic sectional views of first, second, third, fourth and fifth embodiments of the invention with associated circuitry;

Fig. 3 is a cross-sectional View of the input helical wave-guide section for the embodiment illustrated in Fig. 2; and

Figs. 5 and 6 are equivalent circuit diagrams.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in Fig. l, one embodiment of the impedance wall amplifier tube. An envelope 2, whch provides the necessary evacuated chamber, consists of a long cylindrical structure with an enlarged portion at the left extremity as viewed in the drawing. Within the enlarged portion of envelope 2, there is an electron gun 3 comprising a cathode 4 with a heater 6, a focusing electrode 8, an anode 10, and an electrode 12. Heater 6 is connected across a source of potential, such as battery 16, the negative terminal of which can be connected to cathode 4, as shown. Cathode 4 and focusing electrode 8 are connected together and are, in turn, connected to ground.

Anode 10 is connected to the movable arm of a potentiometer 24 which is connected across a source of potential 26, which has its negative terminal connected to ground and its positive terminal connected to electrode 12. Potentiometer 24 is used for controlling the current of the electron stream, a potential of 500 volts with respect to ground being representative of the potential normally applied to anode 10, while the potential applied to elecfrode 12 is of the order of 1000 volts.

An input helix 30 is maintained in a fixed position by evacuated envelope 2, its left extremity being connected by a lead 14 to electrode 12, while its right extremity is terminated in the vicinity of 38 by a coating 3l of absorbing material, such as aquadag, applied on the outside of envelope 2 about the last few turns of the helix 30. Helix 30 is axially aligned with the electron stream emitted from electron gun 3, and has an inner diameter that is substantially equal to the inner diameter of the electrode 1 2 so that the stream electrons pass as close to the helix as possible without being intercepted by the latter. A material, such as tungsten, is suitable for making the helix, the main prerequisite being that it retain its form, especially with res'iect to its pitch and diameter.

An input waveguide 34 is mounted so that lead 14, connecting input helix 30 to electrode 12, is located approximately one-quarter wavelength from the shorted end of the waveguide, as indicated in the drawing. Lead 14 is'also placed so as to be parallel to the electric field in the waveguide 34. A matching element 36 provides a means Y .K 5* N... ..i.* H. -v,-- toadjust thedistance from lead 14 to thejend gof the input waveguide 34 s o that the voltage inducedV in lead 14 can be adjusted to an optimum value.`

Cylindrical collar 18 is concentric with electrode 12 for a distance of roughly one-quarter wavelength. Since the stub formed by collar 18 and electrode 1,2,is open circuited at the far end with respect to the waveguide, an apparent shorting plane is produced atrthe inner surface of the zwaveguide. n

:The impedance member 38 is tubular in form and axially aligned with the input helix,rt he inside diameters of the two being approximately equal. Member 38 may be made of glass with a resistive. coating of vtin, oxide disposed on its inner surface. Resistive coatings of this type are generally thinner than the skin depth of a radiofrequency wave in a lossy ,dielectric material, hence the radio-frequency resistance of the resistive coating approximates its direct-current resistance. Thus, a resistive coating of tin oxide is equivalent to theresistive surface of a lossy dielectric material at frequencies where the f desired operating frequency, the velocity of the electron input waveguide. A cylindrical collar 45 runs concentric withrelectrode 46 for a distance of approximately onequarter wavelength producing an apparent shorting plane on the inner surface of output waveguide 52. Although a rather specic means has been disclosed for coupling signal energy to and from the disclosed impedance wall amplifier tube, other methods, such as replacing the input and output waveguides with coaxial lines, may be used. Also, as will be described later, the input and output helices can be replaced by resonant cavities or a grid can be used to directly modulate the stream of electrons at the input, and a screen and anode similar to those of a tetrode can be used to extract the signal energy from the electron stream at the output end. In addition, the disclosed invention need not be restricted to the particular geometrical configuration described as the electrons ycan be made, for example, to ow along any desired curved path merely by the utilization of appropriate electrostatic and magnetic fields.

g of 500 gauss running axially with the length of the tube.

stream and the dielectric constant of the material onv sistance per square centimeter of a sheet of thematerial having a thickness equal to the skin depth. The choice of surface resistivity ofthe wall required for obtaining maximum amplification per unit length of the tube depends upon the operating frequency, the separation between the electron stream and resistance wall, the electron velocity, and the dielectric constant of the wall. In normal practice, values of surface resistivity will he found to range from 500 to 10,000 ohms per square centimeter. A typical value of surface resistivity for the inner surface of member 38 is of the order of 1000 ohms per square centimeter. The coated glass member may be replaced with other resistive elements, such as a carbon tube or other thick tubular structures possessing the correct surface resistivity. The member 38 is usually maintained at a potential approximatelyequal to that of the accelerating electrode 12. For convenience, the same battery 26 is usedto apply a potential of the order lofl000 volts with respect to ground through leads 40 and 42 to both ends of member 38. t Y t The output portion of the impedancerwall amplifier tube comprises an output helix 44 connected by a lead 20 to an electrode 46 anda collector electrode 47, all maintained at a potential approximately equal to the potential of electrode 12. Again, for the sake of convenience, the same potential source is used to impress a potential of the order of 1000 volts with respect to ground `on output helix 44 and electrode 46 through lead 48, and to impress a like potential on collector electrode 47 through lead 50. As was mentioned in connection with the input helix 30, tungsten is a suitable material out of which4 to make the output helix 44. The helix 44 may be terminated in a manner similar to helix v by applying a coating 43 of absorbing material such as aquadag on the outside of envelope 2 around the first few turns of helix 44 adjacent to member 38.

The construction of the output waveguide 52 is iden.-V

The purpose of this magnetic field is to keep the electron stream focused or constrained throughout the length of the tube. Y

In its operation, an input microwave signal is applied through input waveguide 34, inducing a signal potential on the lead 14 connecting the input helix 30 to the electrode 12. The matching element 36 is adjusted to give maximum signal voltage at the input helix 30. As in conventional traveling wave tubes, the axial phase velocity of the traveling wave through the helix is determined by the pitch and diameter of the helix and is a fraction of the velocity of light. The velocity of the electron stream is usually adjusted so that it is slightly greater than the phasevelocity of the wave in passing through g the helix. The interaction of the electron stream and the wave on the helix results in a density and velocity modulation of the electron stream. The modulated electron stream after passing the helix enters the tubular member 38. The exposed inner surface of member 38 has resistance and dimensions such that a propagated electric Wave is very highly attenuated in the absence ot' electrons. Member 38 is referred to a non-propagating structure insofar as it will attenuate a signal by of the order of 40 decibels as compared to 3 to 5 decibels attenuation by the helix of a conventional traveling wave tube. It is also to be noted that, due to the electrical discontinuities between helices 38, 44 and the member 38, the overall attenuation in the absence of the electron stream from input 34 to output 52 of the tube of Fig. 1 is of the order of decibels.

Hence it is-seen that nearly all of the energy transmitted along the axis of the member 38 is in the form of a space charge Wave propagated by the electron stream. Signal energy exists in a space charge wave in the form of electron bunching. Axial current induced by the hunched electron stream in the walls of member 38 produce electric fields which act on the stream electrons so as to increase still further the electron bunching. An increase in electron bunching is equivalent to an increase in the signal amplitude. Thus, the magnitude of signal on the electron stream continuously increases as the stream electrons move along contiguous to the resistive surface of member 38. As the amplified space charge wave emerges from the far end of the member 38, it induces voltage on the helix, thereby transforming a portion of the signal energy in the electron stream into the form of an electromagnetic wave on the output helix. The stream electrons, as they proceed toward electrode 46, continue to impart energy to the growing wave on the helix. After transferring most of the signal power from the stream tothe4 helix, theelectrns finallyare collected in collector elec;

7 trode 47. The electric field induced by the electromagnetic wave on lead 2t) of the output helix is parallel with the electric field of the fundamental mode desired to be excited in the waveguide; hence, amplified signal energy is transferred from the output helix 44 to the output waveguide 52.

An analysis of the problem of obtaining maximum gain in an impedance wall amplifier tube indicates that the electrons should pass as closely as possible to the walls of the amplifying member 3S, the gain being a maximum if it were possible to pass electrons through a solid medium. In actuality, this is not possible, but an approximation may be made by visualizing the electrons in the form of streams going through a myriad of small holes in the solid medium.

For a current density and an el `tron velocity within the ranges encountered in practice, is possible to obtain appreciable amplification in this type of tube in which the electrons are projected through a medium having conductivity and an absolute dielectric constant, (eoel). For example, at a current density of 0.1 ampere/square centimeter', with an average electron velocity of 1000 volts, and using optimum value of conductivity, the estimated gain is of the order of l decibel per centimeter length of the amplifying section.

Not only is the gain quite high at the frequency of optimum operation, but high gain is available over a wide requency band. For example, calculations Show that in a tube with a theoretical electronic gain of 40 decibels, having an estimated overall gain of 35 decibels, the arnplification would be down to 32 decibels at 5l percent and at 151 percent of optimum frequency, thc tube having a bandwidth of 102 percent. Optimum frequency, o, is defined as the frequency at which the quantity attains its optimum value (equal to about \/3 on the basis of elementary theory) with fixed values of conducticity, a, and absolute dielectric constant (enel). Again from the elementary theory, the maximum gain at the optimum frequency is (decibels) Gain -=8.7 centimeter .In ampere/square centimeter) 3 @Vo/2 (kilovolt-sy/2 wherein J is the current density of the electron stream, Vo the potential through which the stream electrons are accelerated` and eI the real part of the relative dielectric constant. lt is apparent from the foregoing analysis that the electron stream in a resistive medium, as described in the disclosed embodiment of thc impedance wall amplifier tube, is capable of supporting a wave which will greatly amplify initial modulation on the stream over au exceedingly wide frequency band. At the same time there is no critical requirement on the electron stream velocity as is the case in conventional traveling wave tubes.

An additional embodiment cf thc disclosed invention is illustrated in Fig. 2. An env-clope 70 furnishes the necessary evacuated chamber required for the tube. Evacuated envelope 7i) consists of a long tubular structure with an enlarged portion at the left extremity as viewed in the figure. A circular source of electrons, including a cylindrical cathode 72, a circular electronemitting surface 73, and a heater 7i., is axially aligned with the long tubular portion of the envelope. Heater 71 is connected across a source of potential 74, which has its negative terminal of which is connected to cathode 72. Cathode 72 is maintained at a negative potential with respect to ground by a source 76. The potential of source 76 may be of the order 11000 volts.

An input helical waveguide section 78, positioned axially in line with cathode 72, is provided with a cylindrical path 79 for the electrons emitted by cathode 73. The input waveguide is illustrated on an enlarged scale in Fig. 3, where the inner portion of the guide is illustrated in side elevation. The cylindrical slit or path 79 is axially aligned with cathode 72. An input waveguide is connected to the end of waveguidc "/'3 nearest t0 cathode 72. A seal 77 retains the vacuum within envelope 70.

A tubular member 80, having a resistive surface, is positioned axially in line with the cathode 72 and slit 79, member 80 having an inside diameter slightly greater than the outside diameter of slit 79. Member 30 comprises a glass tube, the inner surface of which is provided with a resistive coating of tin oxide, for example. Methods for depositing a resistive coating are known, and, hence, further description would be unwarranted. A suitable surface resistivity for tubular member 30 is of the order of 1000 ohms per square centimeter. Other materials having the specified resistivity can be used, the main consideration being that the stream electrons pass in the near proximity of a surface or wall having a resistivity in the range previously specified for the particular frequency and dielectric constant used.

An output helical waveguide section 82, similar in shape and form to the input helical waveguide section '73, is positioned axially in line with tubular member 80 at the opposite end of envelope tube 70. Both output and input waveguides are made of a nonferrous conducting material, such as copper, silver or silverplated copper.

An output waveguide S4 is connected to the output helical waveguide section 82 on the side nearest to a circular collector electrode 86 which is disposed so as to intercept the electron stream. A seal 85 retains vacuum within the tube. The waveguides 78 and 82, and tubular member 80 may all be maintained at a common ground potential by appropriate ground connections thereto so the average velocity of the stream electrons after their initial acceleration will be unaffected. Collector electrode 36 is maintained at a potential that is sufficiently positive with respect to the potential of the output waveguide 82 so as to suppress any secondary emission due to the stream electrons impinging on the collector electrode surface. An appropriate potential to effect this result is applied to collector electrode 86 by means of a connection to a potential source, such as a battery 87.

A solenoid S8, positioned symmetrically about the complete length of the envelope 70 and connected t0 a source 90, produces a magnetic field parallel to the axis of the tube having a flux density of the order of 500 gauss. As before, the purpose of this magnetic field is to keep the electron Stream focused or constrained throughout the length of the tube.

Since the theory of operation of the tube, illustrated in Fig. 2, is essentially the same as the theory of operation of the tube illustrated in Fig. l, only a brief explanation of its functioning is necessary. With potentials applied to the tube elements, as specicd, electrons will be emitted from cathode 72 and accelerated toward slit 79. Since circular slit 79 is in alignment with cylindircal cathode 72, a cylindrical stream of electrons will be formed which, due to the axial alignment of all` the elements, will proceed on through circular slit 79, tubular member circular slit 83, to collector electrode S6, where the remaining kinetic energy of. the stream electrons is dissipated. The magnetic field produced by solenoid 88 keeps the circular' electron stream focused or constrained to substantially its original cross-sectional dimensions throughout the length of the tube.

The microwave signal to be amplified is applied through input waveguide 75. The dimensions of the waveguide should be such that the rate of change of phase velocity with respect to frequency is relatively small so as to operate the tube in as broadband a region a possible. For constructive interaction between thc stream and the guided wave, the electron velocity should be just greater than the axial component of phase velocity of the guided wave. In other words, the velocity of #andere the` .stream electrons'fis Aadjustedfbfy.` ,the potentie ,ci battery 76 so that they travel a. distance equalntofthe pitchof the helix while a wave in the waveguide traveling Mat its phase .velocity .progresses one complete revolution. This synchronousrelationship between the stream electrons and the microwave signal enables the stream electrons to be `modulated by an electric ield `of the same phase each time they pass `through a section of waveguide. The slit going through the .center of the waveguide ,has little effect on thetransmission characteristics of the` waveguide. as it is in a region Where the boundary `current is normally zero for the fundamental transverse electric mode ofY propagation.

,Aften-.being modulated with the signal by the,input helilwaveguide section 78, the stream electrons proceed'lon through tubular member 80. The eiiect of the hollow cylindrical electron stream in passing near the resistive surface of the wall ofitubular member Se is essentially the'same as for asolid .electronstream passing through` atubular :member having resistive-walls, as

described in` connection with the tube illustrated in Fig.. l. Currents `are induced in the resistive surfaceand Vthese currents produce electric iields which, act onthe stream electrons to' increase the electron bunching. Since the bunching of the electrons is representative of the ampli- AThe electron` stream containing the space charge wave e of nclleaedamplitude passes through the tubular member 80, and enters the circular slit 83 of output helical ctiori 82.,` flhe ampliiiedfsp'ace chargeflwave edwith the ontputwavgni'de so asto 'excite ave injthe output helical waveguide section 82," eener'gy soy induced being available at output waveguide connection 84. The stream electrons continue 'rough circularslit y83 to impinge `on` circular collectolr'elect'ro'd" 86 whe're'th'eir kineticenergy is dissipated. l .lfhe potential, on collector electrode Y86` may be'nrmaintained slightly positive with.respe ct `to the potefntr ,l,. o"fhelicalwaveguide` section- 82 `to 'prevent sec-` ondary emission effects. -Y

EStillanother embodiment of the disclosed invention isillustrated in Fig.A 4. `The structure` ofthetube is .the same as Yfor Athe tubellustrated in Fig. l except that an inductivewail member 94 is usedin lieu of the impedance niernberlvyhich has ay wallhaving a resistive surface. Ind'uctive wall mernbxer9v4"` canine rnader of an artiiical dielectric niaterial having .resonance in the vicinity of the'A operating wavelength. ,Such a `material canbernade byfiirnbedding` numerous pieces of metal` in a dielectricmateriah. such asthe type `iilustrzptted by Vdipole V,96, In order tourna/ke thew'articial dielectric material prespond toiavbroaderi bandwidth, `the Vdipoles ,can be` cutto dit: ferent ,wavelengthsfrthroughout a specified band of frequeiiieszjand' :randomly `scatteredthroughout v'the dielectricmaterial. p. 1.:

T alternative means of obtaining a distributed in-V ductairice. in inductive Wall` mernber y94 is touse' a thick wall'fm'ade lof .ferritic material. .Ferritic materials will be .defined as ,thoselrnaterials having a relative permeability constant` substantially `greater, thanunity. ,The permeability' v,ofmsorrie ferritic materials has, the character-i tie of going through therequivalentof apparallel 1- ona ee. in rather narrow frequency band, rnalting-it` uryltousealparticular. ferriticrmaterial havinga resnanceirithe region of the frequency spectrum where ampliiication is desired. The width of this narrow fre-A ;enc'yibaiid would determine the bandpassfcharactertoben forH the particulary ferritic,materiai. permeabilityconstant of different fer--V quericies, a brbde'r response `can be obtained-by blend-n ing"two or more of the ferritic materials together hav- `es throughresonance vat ditferentfre-,r

ing Aresonance `characteristics in the vicinityl of theifref quency range desired. The specific ferritic materials used are dependent upon the frequency and bandwidth to be l amplified.

The general functioning of the impedance wall ampliiier tube illustrated in Fig. 4, having an inductive Wall member, is similar to that of the ampliiier tube illustrated in Fig. 1, and, hence, wili not be repeated.

To explain more adequately the advantages of the inductive wall member over the resistive wall member, a comparison will be made between theirv equivalent circuit diagrams. Fig. represents the equivalentcircuit diagram of tubular member 3S which has resistive walls. Y lumped parameter form, the series resistance of member 38. Tubular elements 110, 1H, i12, 113 and lle represent the electrical coupling of the electron stream to the walls. In addition to the foregoing, there is also a distributed series capacitance represented as lumped capacitors 05, 106, 197y and 16E.

The equivalent circuit diagram of inductive wall niember 94 is shown in Fig. 6. As before, resistors 163, i, 102 and 103 represent, in lumped parameter form,

the series resistance of the member; tubular elements 11d, lil, U2, H3 and 114 represent the electrical coupling of the electron stream to the walls; and capacitors 105, H36, 107 and 16S represent the distributedl series capacitance. In addition to the foregoing parameters, the inductive wall member has series inductance In adjusting parameters in electron stream tubes foirmaximum ampliiication, essentially the same rules apply as inzvideo ampliiier circuits:y That is, the impedance is maintainedV as high as possible consistent with the frequencybandwidth desired. `Referring to Fig. 5, it can be seen by inspection thaty admittance presented to the electron fstream consists of conductance plus cap-.f-.citiveA To increase the gainrof the tube, it isl susceptance. necessaryto. cancel Vthe capacitive susceptance. One means of accomplishing this is the use of inductance, as

k, showninFig. 6. For this combination, the admittance,

as seen by the electron stream at thelorwer frequencies with. respect to resonance, is now resistive-inductive.F

When'sucha structure is analyzed,.it will give higher gainnelative tov a..comparable resistive-capacitive wall iffthe inductance is.. adjusted to compensate for capacitive susceptance. n v

,.,.Fig...f7,illustrates an additionalembodiment of the in ventio'nwith:electrostaticfocusing of the electron stream..

Ftf what is disclosed in Fig. 7, it follows that the inventl'cinlmay fbe practiced. either 4with or without the magnetic fieldcreated bythe solenoid. All that is necessai-y is thatthe electrons emittedby the cathode are accelerated and, by using afsuitable electron optical system,',afre directed,iin a stream form, in the direction of theA 'input portion of theftube for proper modulation and the ctmcornitant,bunching` of this beam in response to the input signal. This .modulated beam is then made toitravel through the impedancemcmber in the manner described` in connection with Figsl through 6. The main ,difference between Athestructure of this tube and the previously described tubes, therefore, resides in the fact that an electrostatic focusingis used in this case which,l'depending on the electron optics used, may producevaconverging beam illustrated in Fig. 7, or a substantially parallel beam, which generally will have a tendency to diverge because V,of themutual repulsion oi electrons and their thermal velocities. The degree of final ydivergence isa function of the kinetic energy irnpart'ed' to the.,electrons lat thev time they-are vaccelerated v by eacceleration `e1ectrodes.' .-All of the above is well Resistors 100, 101, N2 and 193 represent, in`

fact that the invention is equally applicable to all known types of electron streams so long as these streams have sufficient electron density and initial kinetic energy to be suitable as an amplification vehicle. As will become more apparent from what follows, all that is necessary is to make the shape of the impedance member conform with the shape of the beam. As will be pointed out later, the beam cross-section may also be of e liptical, rectangular or other suitable form.

Referring again to Fig. 7, a cathode 121, mounted in an evacuated envelope 120, has the shape of a cylindri cally concave electron emitting surface for facilitating subsequent focusing of the emitted electrons. If cathode 121 is maintained at ground potential, the potentials of other electrodes are referenced with respect to the cathode. Thus, heater 122 is connected to cathode i2 a intensity grid 125 is at a slightly negative potential with respect to the cathode, and a focusing and accelerating electrode 131 is at a high positive potential so as to impart sufficiently high initial kinetic energy to the stream electrons. Grid 125 is positioned in front of and parallel to the electron emitting surface of cathode 121; it is a cylndrically concave grid having a radius of curvature approximately equal to the distance to thc virtual focal point of the electron stream, so that the current modulation wave fronts, as they progress along the resistive walls, will travel equal distances and subs.- quently impinge on a surface shaped to correspond with the shape of the wave fronts. The actual biasing potential supplied by battery 124 to grid 125 which may be of the order of -2 volts is applied to grid 125 through a parallel resonant combination of an inductor .126, a resistor 127, and a capacitor 128. Resistor 127 is used for flattening the frequency response characteristics of the parallel resonant circuit. An input signal is applied to grid 125 through a capacitor 130.

An electrostatic focusing electrode 131 is positioned symmetrically in front of grid 125, the value of the potential applied to it being of the order of 200 volts.

Since in the illustrated example, the electron stream is a converging and then diverging stream because of the illustrated electron optics, the impedance member 132 is made to conform to the geometrical configuration of the stream. The stream may have a cross-section that is circular, elliptical, rectangular, or other configuration, as previously mentioned.

As stated previously, numerous other geometrical configurations can be used without departing from the spirit of the invention so long as the configuration of the impedance member is such that it has a surface that is contiguous to a boundary of the electron stream.

As described previously, member 132 may be primarily resistive or inductive with the parameters having the same optimum values as in the previous cases. As before, member 132 is maintained at a suitable positive potential with respect to cathode 121 by connecting it over conductor 131 to battery 124 to maintain the velocity of eletcrons in the beam at the optimum value. This optimum potential may be of the order of +100 volts with respect to the potential of the cathode.

A screen 133 and a plate 134 are positioned at the extremity of member 132 so as to intercept the electron stream projected between resistive walls of member 132 from cathode 121. Screen 133 and plate 134 have parallel surfaces which are cylndrically concave on the side exposed to the electron stream so as to equalize the total distance traveled by individual electrons of the stream from grid 125 to plate 134. A positive potential with respect to ground is applied to screen 133 by means of a connection to an appropriate terminal of a battery 135. Similarly, a positive potential with respect to ground is applied to plate 134 by battery 135 through a parallel combination of an inductor 136, a resistor 137 and a capacitor 13S, the values of which may correspond to the values of the respective elements 126, 127 and 128 on the input side in order to obtain the same frequency bandwidth in the output circuit as well as in the input circuit. The potential of screen 133 is maintained negative with respect to that of plate 134 so as to suppress secondary emission of electrons from plate 134. The potential applied to screen 133 may be of the order of |120 volts and the potential applied to plate 134 may be of the order of +20() volts. The output signal is available at a terminal 139 which is connected through a capacitor to plate 134.

The amplifier tube illustrated in Fig. 7 functions as follows: The electrons emitted by cathode 121 are accelerated by electrode 131, so as to produce an electrostatically focused electron stream having a rectangular crosssection and narrowing down to a thin sheet at the focal point. The input signal is applied to grid 125 through capacitor 13), grid 125 modulating the electron stream and producing bunching of the electrons or varying the electron density of the beam in accordance with the applied signal. The input signal is applied across the parallel combination of inductor 126, resistor 127 and capacitor 128, which is resonant at the center of the operating frequency range.

The modulated electron stream is then projected between the resistive walls of member 132 whose length is of the order of several wavelengths, i. e., the distances between the crests of modulation along the stream is a small fraction of the length of the resistive Walls of member 132.

As previously explained, the space charge wave propagated by the electron stream increases in amplitude as the stream progresses from the input to the output end of the member 132 along the resistive walls.

The electron stream propagating the amplified space charge wave continues from its entry between the resistive walls of member 132 through screen 133, and finally impinges on plate 134, where it produces an amplified signal voltage across the output impedance. The output signal is available at terminal 139 which is connected to plate 134 through capacitor 140.

Although the tube described in Fig. 7 illustrates the use of a resistive wall amplifying section, it can, of course, be replaced by any of the previously described inductive wall amplifying sections without changing the mode of operation.

Fig. 8 illustrates a tube of the present invention wherein the use of resonant cavities for coupling the input and output signals to and from the electron stream, respectively, is illustrated. As in the previously described tubes, an electron gun, including cathode 143, focusing electrode and accelerating and focusing electrode 146 for producing an unmodulated electron stream, is positioned in the left extremity of an evacuated envelope 142. The tube also includes the impedance member 159 which may be either a resistive or an inductive wall element of the type described in connection with the prior tubes, a resistive coating 149 being illustrated in the described tube. A collector 155, a solenoid 156, and a source of positive potential 152, are also provided, the latter keeping collector 155 and the resistive coating 149 at the proper positive potentials. An input resonant cavity 150 is positioned between electrode 146 and the left extremity of impedance member 159, as viewed in the drawing. This cavity resonator represents a ring-shaped member of circular configuration when the beam produced by the electron gun has a circular cross-section and is elongated either in the vertical or horizontal plane, if the electron beam produced by the gun has a larger dimension in one plane than in the other. The same type of resonant cavity 151 surrounds the electron stream exterior to envelope 142 at the right extremity of the tube. Both cavities 150, 151 are tuned to resonate at the center frequency of the input signal frequency range. The input cavity is excited in the conventional manner by means of an input loop 153. Useful output signal appears on the output loop maar;

similarly to the previously described tubes having axial magnetic fields, except for the input and output coupling means to the electron stream. A signal is applied to input.

loopY 153 which excites resonant cavity 150, thereby to produce an electric field across the circular slit 157 which penetrates sufficiently far into the electron stream to modulate it in accordance with the applied signal. As in theprigr embodiments, the modulations of the electron streamv arel amplified in passing through impedance mem-1 ber- 159 becausecof the interaction between the modulations propagated by the electron stream and electric fields prpduced by currents induced in resistive coating 149 by` the modulations. As the modulated electron `stream emerges from the portion of the tube having resistive coating 149 and passes the circular slit 158 of the output resonant cavity. 151, electric fields arev generated across the circular slit 158 by the modulation of the stream,

setting up an electric field within the" cavity corresponding tofthe applied signal. Output loop v154 is used to remove the signal energy from the output cavity 151;" '5" Since resonant cavities usually have an appreciable capacitance, the frequency response of this tube, as a whole, may be narrower than the frequency response of theamplifying section alone. However, because of higher impedances of resonant cavities for a given electron stream," tubes using cavities will produce a higher overall gain yatr the center frequency than the impedance wall tubes previously described. It is obvious that any type of im pedance Wall structure, described previously, can be used in connection with the resonant cavity mode of coupling, and hence their use in combination with the described input and output resonant cavities is encompassed by this invention. f

In summarizing the principles of operation of the disclosed embodiments of the present invention, it is seen that in every case a modulated electron stream is projected contiguous to the surface of a non-propagating impedance member for a minimum distance of several wavelengths thereby increasing the modulation of the stream. The surface of the impedance member may present a resistive-capacitive, resistive, resistive-inductive, or inductive impedance to the electron stream. In the case of the resistive surface, the resistance may be provided either by a material having lossy dielectric characteristics or by a resistive coating on glass. ber is non-propagating in the sense that, in the absence of the electron stream, it highly attenuates electromagnetic waves having a free space wavelength in the operating range of the tube. The increased modulation of the electron stream is then converted to a useful amplified signal in the output circuit. Although the helical means have been described with which to modulate and remove signal energy from the electron stream, which is a preferred method in tubes designed for very high frequencies of the order of thousands of megacycles. The specifica tion also discloses other means, such as resonant cavities in one case, and grid structure in another case, which also are available for performing these functions at lower frequencies in combination with the disclosed method of amplification Therefore, the invention is not restricted to any specific type of input or output circuitry that may be used. What is essential for practicing the disclosed invention is to pass the modulated electron stream adjacent to a lossy material surface, either of resistive or inductive type, for increasing the modulation and thus produce the desired amplification in the overall system.

The impedance mem- 1. An electron stream tube for amplifying a microwave signal, said tube comprising means for producing an electron stream, means for modulating said electron stream with the microwave signal to launch a concomitant space charge wave Vpropagated by the electron stream, means fdrfdirecting said modulated electron stream along a predetermined path, a`substantially non-propagating membei-Shaving a surface disposed contiguously along said predetemiined path for `a minimum distance of several wavelengths of said space charge wave, said member pre; senting an impedance to" said stream at said surface tofenablesaid space charge wave to induce electric 'currents in saidmember which' generate electric fields which, in'turn, interact with said electron stream to produce amplification of said space charge wave, and output means coupled to said electron-stream for converting said ampliiiedfspa charge ywave to a microwave output signal.

`2." An electron stream tube for amplifying a microwave signal, saidtube comprising anevacua'ted envelope, means fo'rproducing an electron-stream within said envelope, inpu'tfnleans` for 'modulating said electron stream with a microwave input signal, means for directing said modulated electron stream along a predetermined path within said envelope, a substantially non-propagating member includinga wall having a' surface'extending contiguously along said pathk for a distance of at least several modulationsof saidelectr'lon stream, said wall presenting an irnpedancetoE said Aelectronstream to amplify said modulations, and output means coupled to said electron stream for converting said amplified space charge wave into an electromagnetic output signal.

3. The electron stream tube as defined in claim 2 wherein the surface of the wall of said substantially non-propagating member has a surface resistivity in the range of from 500 to 10,000 ohms per square centimeter.

4. The electron stream tube as defined in claim 2 wherein said substantially non-propagating member including a wall having a surface comprises a glass structure having a surface, and a resistive coating disposed on the surface of said glass structure.

5. The electron stream tube as delined in claim 4 wherein said resistive coating is of tin oxide.

6. The electron stream tube as defined in claim 2 wherein the wall of said substantially non-propagating member is composed of a dielectric material having a plurality of dipole elements distributed throughout said dielectric material.

7. The electron stream tube as defined in claim 2 wherein the wall of said substantially non-propagating member has a conductivity substantially equal to weoelx/S where w is the angular frequency of the signal frequency range, e0 is the absolute dielectric constant of free space, and el is the real part of the relative dielectric constant of said wall.

8. The electron stream tube as defined in claim 2 wherein the wall of said substantially non-propagating member is composed of ferritic material for making the impedance inductive presented to said electron stream at the surface of said wall.

9. A wave-type amplifier tube comprising means for producing an electron stream, means for modulating said electron stream with a microwave input signal, a nonconducting tubular element disposed concentrically about and contiguous to said modulated electron stream, a resistive coating disposed on the inner surface of said element, whereby the initial modulations of said electron stream induce electric currents in said resistive coating which generate electric fields which, in turn, interact with said electron stream to amplify the initial modula tions thereof, and output means coupled to said electron stream for converting the amplified modulations of said electron stream into an electromagnetic output signal.

10. An electron stream tube for amplifying microwave signals, said tube comprising means for producing an electron stream, means for directing said electron stream along a predetermined path, input means responsive to a microwave input signal for modulating said electron stream, a non-conductive tubular member disposed concentrically about said predetermined path, the inner surface of said member being contiguous to said electron stream, a resistive coating disposed on said inner surface, whereby the modulations of said electron stream induce currents in said resistive coating which generate electric fields which interact with said electron stream to effect amplification of said modulations, and output means for converting the amplified modulations of said electron stream into an electromagnetic microwave output signal, said input means, said tubular member, and said output means being disposed along said path in the direction of electron flow in the order named.

11. The electron stream tube as defined in claim 10 wherein said tubular member additionally includes means for cancelling the spatial capacitance immediately adjacent said resistive coating.

12. An electron stream tube for amplifying a microwave signal, said tube comprising means for producing an electron stream, means for modulating said electron stream with a microwave input signal to launch a concomitant space charge wave propagated by said electron stream, means for directing said modulated electron stream along a predetermined path, a non-propagating member having a surface disposed contiguously along said predetermined path for a minimum distance of 16 several wavelengths of said space charge wave, said member presenting an impedance to said electron stream at said surface, said member being substantially the only means providing amplification of said space charge wave, whereby said space charge wave induces currents in said member which generate electric fields which, in turn, interact with said electron stream, and output means coupled to said electron stream for converting the amplited space charge wave into a microwave output signal.

References Cited in the file of this patent UNITED STATES PATENTS 2,580,007 Dohler et al. Dec. 25, 1951 2,603,772 Field July 15, 1952 2,603,773 Field July 15, 1952 2,611,102 Bohlke Sept. 16, 1952 2,626,371 Barnett et al. Jan. 20, 1953 2,652,512 Hollenberg Sept. 15, 1953 2,654,047 Clavier et al. Sept. 29, 1953 2,660,690 Breeden et al. Nov. 24, 1953 2,661,441 Mueller Dec. 1, 1953 FOREIGN PATENTS 962,479 France Dec. 12, 1949 OTHER REFERENCES Article by Kompfner, The Traveling Wave Tube as Amplifier at Microwaves. Proceedings of the I. R. E., 1947, page 124 thru page 127. 

